The present invention relates to a waste heat recovery device for a boiler or the like and, more particularly, to a waste heat recovery device which prevents the heat conduction surfaces of a heat exchanger from corrosion by sulfur oxides contained in exhaust gas.
In a boiler or an industrial furnace which uses a fuel such as a heavy fuel oil containing sulfur, or an incinerator which exhausts a sulfur oxide SO.sub.x, a problem has been noted in that the heat conduction surfaces of a water heat exchanger are corroded by sulfur oxide SO.sub.x in the exhaust gas. The surface temperature of a heat conduction coil of the typical water heat exchanger is slightly higher than the water temperature. Therefore, even if the temperature of the exhaust gas flowing through a duct is higher than the dew point of the sulfur oxide SO.sub.x (the dew point may vary in accordance with the sulfur content of the exhaust gas, but is substantially 140.degree. C.), the temperature of the exhaust gas is lowered when it comes in contact with the surface of the heat conduction coil. The sulfur oxide SO.sub.x contained in the exhaust gas condenses on the surface of the heat conduction coil. As a result, the sulfur oxide SO.sub.x is converted to sulfuric acid and then corrodes the surface of the heat conduction coil.
In order to avoid this problem, a waste heat recovery device is provided which utilizes the constant temperature characteristic of a heat pipe which is maintained at the temperature of a heat-receiving side. For example, Japanese Utility Model Publication No. 55-18,641 proposes a waste heat recovery device of this type. However, if this device is installed in a boiler or the like having load fluctuations, the surface temperature of the heat conduction coil of the heat pipe at the heat-absorbing side changes in proportion to a change in temperature Tg of the exhaust gas. With a decrease in the temperature Tg of the exhaust gas, a surface temperature Tm of the conduction coil at the heat-absorbing side is lowered below the dew point of the sulfur oxide SO.sub.x, as shown in FIG. 1. Therefore, the surface of the heat conduction coil is corroded by the sulfur oxide SO.sub.x, the coil is blocked, and service life of the pipe is degraded. (Referring to FIG. 1, Tgi denotes the temperature at an inlet port; Tgo, the temperature of the gas at an outlet port; Two, the temperature of water at the outlet port; and Twi, the temperature of water at the inlet port.) Furthermore, the temperature Tg of the exhaust gas becomes lower than a predetermined level due to the heat absorbing action of the waste heat recovery device. The problem in which the duct or the inside of the chimney is corroded by the sulfur oxide SO.sub.x has not yet been solved.